The present invention relates generally to a measurement method, and more particularly to a measurement method for measuring an optical characteristic of an optical system. The present invention is suitable, for example, for a measurement of a wavefront aberration of a projection optical system that is used to project a reticle pattern onto a plate.
A conventional projection exposure apparatus manufactures a fine semiconductor device, such as a semiconductor memory and a logic circuit, using the photolithography technology. The projection exposure apparatus projects and transfers a circuit pattern of a reticle or mask onto a wafer etc. through a projection optical system.
The projection exposure apparatus should transfer a reticle pattern onto a wafer at a preset magnification or reduction ratio. It is important for this demand to use a projection optical system that has an excellent imaging characteristic with an extremely reduced aberration. In particular, the recent, rapidly promoting fine processing to semiconductor devices faces a problem of deteriorated pattern resolving power resulting from an aberration in manufacturing and designing the projection optical system. A fine pattern is sensitive to the aberration of the optical system. It is thus necessary to precisely measure an optical characteristic or aberration of the projection optical system.
Various measurement methods of the aberration of the projection optical system are currently proposed, and an actual evaluation and inspection of the projection optical system utilize measurements of various types of aberrations, such as a spherical aberration, a curvature of field, an astigmatism, a coma, and a wavefront aberration. Among these aberrations, the wavefront aberration is the very aberration. Once the wavefront aberration is approximated by the Zernike polynomial, other elemental aberrations can be calculated, such as a spherical aberration, a curvature of field, an astigmatism, and, a coma. In general, these aberrations are low frequency aberrations, and expressed by the fourth term to thirty-sixth term in the Zernike polynomial. A higher frequency aberration has never mattered little.
Along with the recent resolution enhanced technology, it is discovered that a high frequency component of the wavefront involves at least one of various problems near the resolution limit. A different aperture area around the reticle pattern typically leads to a critical dimension (“CD”) difference of an exposed image. This phenomenon is caused by the light rounding a pattern, similar to the flare, when the aperture is large around the pattern. The flare is called a stray light and degrades imaging. The flare results from reflections on all members around a lens barrel and the lens barrel itself that holds a lens in a system in an exposure apparatus that includes the lens and a reticle. This phenomenon attributes only to the lens's wavefront. In general, the aberration is referred to as local flare. It is known that the local flare does not occur due to a low order aberration, but occurs due to a high order aberration or high frequency wavefront shape.
A method using a phase measurement interferometer (“PMI”) is one local-flare quantifying method. This method analyzes PMI measured wavefront data through a fast Fourier transform (“FFT”) frequency analysis, and expresses the intensity in a specific frequency range with a root mean square (“RMS”) converted numerical values. The PMI can measure comparatively high order aberrations, and thus can measure the local flare at a high precision. Other known methods are a Kirk method and a method that actually transfers patterns having different aperture areas to a resist, and expresses the local flare with a pattern CD difference. The Kirk method exposes an aperture part having a predetermined area and a comparatively large BOX mark (with several micrometers) arranged in its center on plural shots while changing an exposure dose. The Kirk method represents the local flare with a ratio between an exposure dose at which the resist image of the BOX mark disappears and an exposure dose at which the resist image at the aperture part disappears.
The PMI local-flare measurement is viable to an adjustment and an inspection of a single lens, but is usually inapplicable after the lens is installed onto an exposure apparatus because the high frequency component's amplitude is very small. It is necessary to improve the measurement sensitivity and pixel resolution in the XY directions of the light receiving part for processing an interference pattern image so as to make the PMI measurement available in the exposure apparatus. Thus, the PMI measurement is a significant burden on the exposure apparatus.
The method of measuring the CD difference on an image transferred on a resist significantly depends upon the resist characteristic and the focusing and exposure dose conditions. Since the pattern's CD error is directly influential, the method needs a correction while taking a proximity effect into account, such as process influence, instead of a mere correction of the pattern CD difference, making the correction very complex. Thus, disadvantageously, this method cannot provide a sufficient precision. Also disadvantageously, the Kirk method degrades the precision due to unclear determinations of the pattern disappearance. These two methods have difficulties in correlating the local flare amount with the PMI measurement due to the above various error factors.
The most preferable measurement method would be to calculate the local flare in the exposure apparatus by measuring the high frequency wavefront shape, similar to the PMI, as disclosed, for example, in U.S. Pat. Nos. 5,828,455 and 5,978,085. This measurement method uses a special reticle that has a grating pattern arranged on a patterned surface, a pinhole below and apart from the grating pattern, and a convex lens above the grating pattern on a reticle glass surface opposing to the patterned surface. The illumination light has an angle of σ of 1 or greater due to the convex lens, and illuminates the grating pattern. The light that passes the grating pattern passes the pinhole below it. The light that can pass the pinhole is limited to the light having an angle formed by connecting each grating pattern position and the pinhole. In other words, the light emitted from each grating pattern has a different angle. The angularly different light reaches a different position on the pupil plane, and is incident or images on the wafer plane under influence of the wavefront aberration of the projection optical system. Each transferred image is subject to influence of a different wavefront aberration or phase. Since the light travels in a tangential direction of the wavefront, each transferred pattern shifts by a wavefront's inclination. The wavefront aberration can be calculated through various mathematical approaches by measuring a shift of the transferred pattern image from an ideal grating to obtain the wavefront's inclination in the pupil plane in the projection optical system.
A measurement method that utilizes an obliquely incident illumination is also conceivable. See, for example, Japanese Patent Application, Publication No. 2004-146454. This measurement method illuminates a reticle pattern with illumination lights having principal rays with different inclined angles or incident angles. The principal ray of the pattern illuminated by illumination lights having different angles or directions reach different positions on the pupil plane in the projection optical system. Similar to U.S. Pat. Nos. 5,828,455 and 5,978,085, the wavefront can be calculated by measuring a pattern's positional shift for each illumination angle and direction. These methods detect a wavefront's slope, and calculate a wavefront aberration. These methods have an advantage in a higher sensitivity to the high order aberration because they can more easily detect a slope even with a small phase difference as the frequency becomes high than the PMI that directly measures the phase.
Another proposed method uses a reticle that has a repetitive pattern having a phase difference of 90° and a pitch within a certain range, and forms an image through the two-beam interference, i.e., between the 0th order light and the 1st order diffracted light. Then, this method calculates a phase difference from a positional shift for each image pitch size, and directly calculates a wavefront.
None of the above three wavefront-aberration measurement method precisely measures a high order aberration to be recently measured. More specifically, each method forms an image of a reticle pattern, and calculates a wavefront shape from the pattern's positional shift. These methods uses one positional measuring mark with a predetermined size to measure a wavefront at a predetermined pupil coordinate position in the pupil plane. The predetermined size depends upon an NA of the projection optical system, but is about 1/20 as large as the pupil diameter when converted into the pupil plane at an NA of 0.85. This can sufficiently maintain a required precision for a low order aberration up to the 36th term in the Zernike polynomial, but needs a higher resolution when measuring such a high order aberration as that for the PMI. One positional shift measuring mark also needs a higher measuring sensitivity. The measuring sensitivity corresponds to a beam size when the beam is diffracted from one positional shift measuring mark and passes the pupil plane. Thus, a beam diameter should be made small.